1. Field of the Invention
The present invention relates in general to boiler design, and in particular to a new and useful arrangement for the convection pass enclosure of a tube boiler.
2. Description of the Related Art
The design of convection pass enclosures for subcritical pressure drum and once-through boilers and supercritical pressure once-through boilers has consisted of vertical tube enclosure panels that are typically arranged to flow in either an upward or downward direction. The design requirements for these enclosures typically requires that a sufficient inside tube fluid velocity exist at all loads so that tube overheating, fatigue or failure is not encountered during the normal operating life of the boiler.
The two pass type of utility boilers has convection pass enclosures that are located adjacent the furnace enclosure. The pendant convection pass enclosure connects the furnace enclosure to the horizontal convection pass enclosure. The pendant convection pass enclosure is an enclosure that allows the furnace gases to flow from the furnace enclosure horizontally across pendant heat transfer surfaces (vertical arrangement of steam and/or water cooled tubes). The horizontal convection pass contains horizontally arranged heat transfer surface (steam and/or water cooled tubes) that absorbs heat from the furnace gases flowing in a downward direction in the enclosure.
In all two pass type utility boilers, the convection pass enclosure tubes must be designed to remove any stagnate water or steam legs that either exist during startup or that can form at any operating load of the boiler. In many cases multiple flow paths, small diameter orificing of the headers and/or the use of thick tubes with small inside diameters within the enclosure""s circuitry are required to satisfy these requirements. For many boiler performance conditions, the heat transfer surface inside the convection pass enclosures requires an enclosure surface area that utilizes multiple flow paths. For these applications, the multiple flow paths will require a tube size and thickness that places a large pressure drop into the overall system design, which results in a penalty on unit efficiency. To obtain a realistic design, the large pressure drop of the enclosure must be reduced at the higher loads by using a flow bypass to reduce the velocity in the enclosure tubes at the higher loads.
The spiral tube furnace enclosure design was developed by Benson in 1927. This technology has been applied to furnace enclosure design for different reasons than those proposed in this disclosure for the horizontal spiral tube convection pass enclosure design. The concept for the prior furnace enclosures was to provide minimum temperature and enthalpy variation in the furnace enclosure while maintaining a flow velocity across the flow range that will eliminate tube failure resulting from critical heat flux. The concepts of the present invention differs from the Benson concept in that a reduction in the pressure drop of the fluid through the enclosure at higher loads is desired so that flow bypassing and/or thicker tubes and small orifices are not required.
Also see U.S. Pat. Nos. 5,934,227 and 5,755,188.
An object of the present invention is to use a spiral horizontal arrangement of tubes to eliminate the multiple flow passes, the small orifices and/or the thicker tubes of the prior art in a convection pass of a boiler. The spiral design allows flexibility of determining the number of tubes and the number of spiral loops around the enclosure to obtain the optimum velocity that will eliminate any stagnate legs of water or steam during startup and low load operation while still giving acceptable pressure drop at full load without the need for an enclosure bypass.
The concept has been used on three walls of the horizontal convection pass of a boiler. This concept could also be incorporated into both the horizontal and pendant convection passes of the boiler, if needed for performance improvements. The horizontal spiral tube of the invention in this embodiment starts on one of the side walls and encompasses any combination of exterior walls of the enclosure in the lower portion of the horizontal convection pass. If all four walls of the horizontal convection pass are used, the upper section of the horizontal convection pass would only encompass the side and rear walls of the enclosure.
The horizontal spiral tube boiler convection pass enclosure design of the present invention includes five advantages over the existing design.
1. An optimum tube size, tube thickness and the number of tubes for the convection pass enclosure can be determined so that excessive convection pass pressure drop and flow biasing around the convection enclosure is not required for some boiler loads.
2. Material weight of the convection pass enclosure can be reduced through the use of thinner tubes. The thinner tubes can be used to meet the design requirements of eliminating a stagnate leg of water or steam at start up and minimum load.
3. Performance efficiency penalties due to increased pressure drop of the enclosure design would be minimized by incorporating the concepts of this invention.
4. Manufacturing and construction costs of this invention would be slightly more than the current vertical tube design with a flow bypass system, but the performance advantages of the invention would make the addition cost attractive to the market.
5. This invention can be applied to both subcritical drum boilers as well as subcritical and supercritical once-through boilers.
Accordingly, a further object of the present invention is to provide a convection pass enclosure for a boiler comprising a front wall having a gas inlet for receiving gases from the boiler; right and left side walls connected to the front wall; a rear wall having a gas outlet, the rear wall being connected between the right and left side walls; and a roof, at least one of the front wall, right side wall, left side wall and rear side wall having a heat exchanger surface in the form of a horizontal spiral tube assembly.
A further object of the present invention is to provide more than one of the walls of the convection pass enclosure with horizontal spiral tube heat exchangers.